This invention relates generally to single-stage hypersonic vehicles and more particularly to propulsion systems for single-stage vehicles. Hypersonic vehicles are generally characterized as capable of achieving speeds of greater than approximately Mach 5, and have typically relied on rocket engines to achieve such speeds.
Launch vehicles or rocket boosters are primarily used to deliver satellites to orbit or weapons over large distances (ICBMs). However, most of the existing rocket designs are expendable, making them costly for most missions and less competitive in the world launch market. A transatmospheric vehicle (TAV) or reusable launch vehicle (RLV) would be capable of returning to earth to be reused after minimal refurbishment and refueling. TAVs most likely would have aerodynamic and operability characteristics similar to conventional aircraft but have capability of delivering payloads to low earth orbit (LEO). The promise of TAVs is that their reusability would potentially allow them to launch payloads into orbit at much lower cost than current expendable rockets.
Both single-stage-to-orbit and two-stage-to-orbit RLVs using hypersonic technology have been studied by NASA and others to deliver small to medium payloads to LEO, while single-stage hypersonic TAVs are under consideration by DoD and offer the promise of launch vehicle responsiveness and flexibility for military global strike missions and reconnaissance. If single-stage and two-stage TAVs could be operated more like an aircraft and less like an expendable rocket, they would offer the promise of carrying out space operations with greater flexibility and responsiveness than is currently possible with expendable boosters. In fact, for many military missions, including space control and force application, satellite payload deliver of 1,000 to 5,000 lbs into LEO may be adequate, compared to typical payload deliveries of 40,000 lbs (expendable launch boosters) and 65,000 lbs (Shuttle).
Whether for space access or global strike/reconnaissance, launch vehicle responsiveness is one of the most important requirements for a DoD TAV, since launching within minutes or hours of a launch order is critical to mission success. This degree of responsiveness implies aircraft-like levels of supportability and reliability. A military vehicle design capable of being launched on alert from a number of continental U.S. (CONUS) bases or forward operating locations could be very different from a commercial RLV designed for a predictable launch from a single launch facility (i.e., Kennedy Space Center). Other important DoD requirements that drive the TAV design include increased survivability associated with speed and altitude to counter “today's and tomorrow's threats”, and the ability to neutralize time-critical targets. These requirements are driving TAV designs for DoD missions towards single-stage hypersonic vehicles.
Airbreathing propulsion engines have several advantages over expendable rockets, namely, they do not require stored liquid oxygen, which results in smaller and less costly launch vehicles. In addition, airbreathing engines don't have to rely strictly on engine thrust but can utilize available aerodynamic forces, thus resulting in far greater maneuverability. This can also manifest itself in greater vehicle safety since missions can be aborted much easier.
Alternatives to all-rocket propulsion systems include a combination of gas turbine jet engines, ramjets, scramjets and rockets that can be integrated into a combined cycle airbreathing propulsion system. Advanced turbojet engines, such as found in fighter aircraft, rely on compressing the air, injecting the fuel into it, burning the mixture, and expanding the combustion products through the nozzle to provide thrust at much higher specific impulses (Isp) than rocket engines. Turbojets can be used to provide horizontal takeoff—like conventional airplanes—and are currently materials limited to Mach 2-3 so as to prevent overheating and damage to the turbine blades. At this point another form of propulsion engine, called a ramjet, takes over. This is in lieu of undertaking an expensive development of high-temperature gas turbine blade materials technology to increase the maximum upper limit to approximately Mach 3-4. The ramjet engine operates by using a specially designed inlet to scoop up the ram air, slow it down and then compress it while the vehicle is flying through the atmosphere. Fuel is injected into the air, mixed with it, combusted and then expanded through the nozzle to provide thrust in a similar fashion to the turbojet. Ramjet engines operate most efficiently at vehicle speeds beyond Mach 2-3. A ramjet can be readily integrated into a turbojet engine. The turbojet by itself would operate from take-off to ramjet takeover, and the ramjet would then power the vehicle to its velocity limit of about Mach 6. Above this limit the combustion chamber temperature becomes very high, causing the combustion products to dissociate, which in turn reduces vehicle thrust.
To operate at still higher vehicle speeds, supersonic combustion ramjets, or scramjets as they are called, would be employed. Again, fuel is injected, mixed and combusted with the air, but at supersonic speeds, thus necessitating a different fuel injection scheme than that used by the ramjet. As the vehicle continues to accelerate into the upper atmosphere, rocket engines may be required to supplement the scramjet engine(s) for Mach numbers above 10-12. Certainly rocket engines would be required if orbit insertion and maneuvering in space (above Mach 18) were required.
There are two major hypersonic combined cycle vehicle design approaches for access to space, one featuring a single-stage vehicle and the other a two-stage vehicle. Some single-stage hypersonic vehicles rely on a low-speed propulsion system responsible for achieving the speed necessary for ramjet operation, and a high-speed propulsion system that operates as a ramjet or a combined ramjet/scramjet. Such systems, however, have several disadvantages. For example, the low-speed propulsion system typically relies on conventional gas turbine engine technology to achieve ramjet viable speeds (less than approximately Mach 3). The overall vehicle weight is sensitive to the weight of the combined propulsion system. Since development and operational cost typically scale with vehicle weight raised to an exponent, it is important to keep vehicle and propulsion weight to a minimum.
One way to eliminate the ramjet and its weight impact from the high speed flowpath is to close the Mach number gap between turbojets and scramjets. This can be accomplished by extending the maximum Mach number range of the gas turbine (turbojet) engine operability beyond Mach 3 and/or reducing the minimum Mach number range of scramjet operability to below Mach 6. However, a substantial investment on the order of hundreds of millions to billions of dollars may be required to advance the gas turbine art so as to minimize the Mach number gap. An alternate approach is to combine the ramjet flowpath with the turbine engine. This creates synergy since the ramjet can utilize the isolator that is already required by the turbine engine in a typical turbine based combined cycle concept installation, rather than creating a second isolator in the high speed flowpath. Further synergy can also be obtained by combining the turbojet engine afterburner with a ramjet combustor, which will then substantially reduce the length and weight of the combined propulsion system leading to significantly better vehicle performance. Thus, there is a need for a single-stage vehicle having an improved propulsion system that could be used by DoD for rapid response and flexibility to provide global strike/reconnaissance missions and access to space to deliver small to medium payloads into LEO at much lower costs than present expendable rockets (benefits both NASA and DoD).